Pyruvate kinase catalyzes the transfer of a high-energy phosphate group from phosphoenol pyruvate (PEP) to generate pyruvate and ATP, a reaction that is the rate-limiting step of the glycolytic pathway. The PKM1 alternatively-spliced isoform is ubiquitously expressed whereas the PKM2 isoform is normally highly expressed only in embryos and undifferentiated tissues, correlating with high rates of cellular proliferation. Notably, tumors express the PKM2 form to the exclusion of the M1 isoform.